In an electrophotographic machine such as xerographic copier, documents that are copied may vary considerably in the optical density of the paper substrate. In the absence of compensating techniques, a copier or printer designed to reproduce large areas respond to the absolute level of input density and hence, tend to produce unacceptable background when copying non-white background documents.
Particularly for relatively thin documents where images on the reverse side of the portion of the document being copied can show through, problems are incurred in the copying apparatus as it tends to copy the reverse side images in addition to the images being directly exposed to the copier. To compensate for this, often white sheets are placed as a backing to cover or eliminate the show through problem. This has proved to be inadequate as the machine tends to sense these images in any event.
It has been found that the use of black sheets as a backing on thin or semi-transparent stock has the effect of causing uniform gray background which can be eliminated with the "copy lighter" mode. Even where the user anticipates the use of such black sheets of paper, it is often not readily available, requiring considerable down time for the user to either locate the stock or otherwise develop the black paper.
Other approaches to solve this problem of thin originals include the use of sensing systems to control the charge on the developer depending on the quality of the paper sensed at initial stages of the copier operation. For example, it is known to provide forms of automatic background stabilization. The Minolta 350 copier uses a passive developer-bias control that increases bias when the average image potential increases. The 3M "Sensitron" copier increases exposure when the average reflectance across the process decreases. Both of these systems compensate for variations in substrate density for low area-coverage-documents but, undesirably, also compensate for areas of high image density mistaking the image density for the background density. Another example of an automatic background stabilization technique is that found in the Cannon MP-270F copier. For this technique the scanning system performs a brief scan of the document, sensing the background image potential with a built in electrometer. Exposure is then adjusted prior to actual exposure scan. This technique results in undesirable loss of productivity due the extra time required for the process.
Other systems such as the Apeco Electric Eye copier suggests the use of automatic exposure control in which light reflected from the original irradiates a small rectangular slit at the side of the optical system housing. This slit is covered with a sheet of standard white paper and light diffusely transmitted by this paper is then incident on a cadmium sulfide photocell placed outside the housing directly behind the paper covered slit. The output of this cell is used to vary the voltage applied to the exposure lamps and thus, vary the exposure. When the amplitude of the light decreases or increases, the system causes the exposure lamp to change accordingly, compensating for the type of original being copied.
A number of patents have suggested other systems for generating a bias voltage depending on the density of the color and the background. For example, in U.S. Pat. No. 4,372,674, a bias voltage is generated depending on the density and color of the background of the original sensed. In U.S. Pat. No., 4,050,806 there is disclosed a plurality of sensing electrodes to sense the background area and electrostatic image and apply a biasing voltage to the developing electrode in response to the lowest value of the sensed potential. In U.S. Pat. No. 4,306,804, there is disclosed the use of a reference surface for a portion of the original document by reverse optical scanning to sense the electrostatic potential on a photoconductive member and to control the intensity of the light image on the original document or the developing bias voltage. Other systems such as those disclosed in U.S. Pat. Nos. 4,200,391, 4,533,238 and 4,544,258 relate to exposure control systems but not scorotron control in an attempt to leading edge black band.
The systems described above do not always completely or satisfactorily accommodate the affect of the background image. These systems rely on varying the exposure or the bias on the photoreceptor to achieve the desired image cooperating with other adjustments on the system. As a result, although some elimination of the background image occurs, the background image tends to be exposed in any event. Consequently, the copy may be adversely affected as well because of the reduced exposure or change in developer bias to compensate for this effect.